(1) Field of the Invention
The present invention relates to a head for a rotorcraft lift rotor, and also to a rotor and to a rotorcraft having such a head.
(2) Description of Related Art
A rotorcraft conventionally comprises an airframe extending from a nose to a tail boom. In general, such a tail boom may also carry rear stabilizing airfoils such as for example a tail fin and stabilizers.
The airframe may carry at least one rotor serving to provide the rotorcraft with at least part of its lift and possibly also with propulsion. Such a rotor is referred to below as a “lift” rotor, and is sometimes referred to as a “main” rotor by the person skilled in the art.
In addition, the tail boom may include rear airfoils, in particular a tail fin possibly carrying a rotor for controlling yaw movement of the rotorcraft and for generating a lateral thrust force serving to compensate the reaction torque created by the main rotor being driven mechanically. Under such circumstances, the rotor is sometimes referred to as a “tail rotor” given its position relative to the rotorcraft, or indeed as an “anti-torque” rotor given its stabilizing function.
The airframe also has covers arranged under the main rotor. These covers may be removable covers for giving access to a power plant, for example. Such covers are conventionally referred to as “engine covers”.
While the rotorcraft is flying in translation, air flows along the rotorcraft. The slipstream downstream from the lift rotor and from the covers of the airframe is generally disturbed. Such disturbance can then impact against the rear stabilizing surfaces and also against the tail boom of the rotorcraft.
This disturbed flow is usually referred to as a “wake”. Specifically, the wake of an object refers to the zone of fluid that is situated downstream from the object and that presents a modification to its state compared with the flow at infinity.
The impact of disturbances generated by a lift rotor or by the engine covers on the stabilizing airfoils of a rotorcraft can potentially lead to one or more modes of vibration of the tail boom being excited aerodynamically, with such excitation commonly being referred to in the art as “tail-shake”. This excitation presents numerous drawbacks, and in particular:                for crew and passenger comfort;        for fatigue in parts and equipment; and        for the operation of certain systems of the rotorcraft.        
In addition, the air flow over the airframe can separate from the airframe downstream from the lift rotor, in particular downstream from the engine covers. Such separation tends to generate turbulence and consequently to increase the intensity of the aerodynamic excitation on the stabilizing airfoils, and it also enriches the frequency signature of such excitation.
In order to reduce such excitation, a head may be arranged on the top of the lift rotor, as described in particular in Documents U.S. Pat. No. 3,181,815, EP 2 727 832, FR 3 010 973, and DE 199 44 412.
Thus, a head is generally in the form of a cap forming a substantially ellipsoidal surface of revolution.
Furthermore, notches are arranged in a peripheral ring of the head, in particular to avoid impeding pitch, flapping, and lead/lag motions of the blades.
Under such circumstances, a head may include a cap in the form of an ellipsoidal surface of revolution having one notch for each blade of the rotor.
While flying in translation, the head deflects the flow of air downstream from the lift rotor in a downward direction. This air flow is then deflected mainly towards the covers and the tail boom, and no longer towards the stabilizers and the tail fin of the rotorcraft. The tail-shake effect is thus diminished.
Consequently, a head tends to deflect the slipstream downstream from a lift rotor in a downward direction.
In addition, the head tends to limit separation of a slipstream downstream from the engine covers.
Nevertheless, such heads are not always optimized.
Specifically, a head is generally designed to deflect a slipstream downwards and to minimize said separation of the slipstream downstream from the lift rotor fitted with the head. These lift considerations tend to determine the diameter of the head.
Under such circumstances, the head does not have any means for acting on the frequency signature of the slipstreams that are generated, nor for acting on the forces to which the head is subjected. A manufacturer therefore cannot take action on a head of given shape in order to address problems of interaction between the slipstream and the head.